When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, gamma-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to a radiation passing through an object such as the human body, to have a radiation image of the objected stored therein, and is then scanned or exposed with stimulating rays such as a laser beam which cause it to emit light in the pattern of the stored image (i.e., in proportion to the stored radiation energy). The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to an electric image signal and the radiation image of the object is reproduced as a visible image by use of the image signals on a recording medium such as a photographic film or on a display device such as a cathode ray tube (CRT) or the like.
One embodiment of the aforesaid radiation image recording and reproducing system is disclosed, for example, in Japanese Unexamined Patent Publication No. 58(1983)-67240. In the disclosed embodiment, before final read-out is conducted by scanning the stimulable phosphor sheet carrying a radiation image of an object stored therein by stimulating rays which cause the stimulable phosphor sheet to emit light in proportion to the radiation energy stored, detecting the emitted light by a photoelectric read-out means and converting it into an electric image signal, preliminary read-out for approximately detecting the image information stored in the stimulable phosphor sheet is conducted by use of stimulating rays of a level lower than the level of the stimulating rays used in the final read-out. Read-out conditions for the final read-out are adjusted on the basis of the information obtained by the preliminary read-out, and the final read-out is conducted by use of the read-out conditions. The electric image signal obtained by the final read-out is sent to an image processing means and is processed in accordance with the image recording portion of the object and/or the image recording method to obtain a visible image suitable for viewing, particularly for diagnostic purposes. The processed image signal is used to reproduce the visible image on a photographic film or the like.
By the term "read-out conditions" as used herein are meant various conditions affecting the relationship between the amount of the light emitted by the stimulable phosphor sheet at the read-out step and the output of the read-out apparatus, for example, the read-out gain (sensitivity) determining the relationship between the input and the output of the read-out apparatus, the scaled factor (latitude), and the power of the stimulating rays used for read-out.
By the term "image processing conditions" as used herein are meant various conditions such as an emphasis coefficient in frequency response processing, and various characteristic values in a gradation processing.
As described above, the level of the stimulating rays used in the preliminary read-out is lower than the level of the stimulating rays used in the final read-out. That is, the effective energy or intensity of the stimulating rays which the stimulable phosphor sheet receives per unit area in the preliminary read-out should be lower than the effective intensity of the stimulating rays used in the final read-out. In order to make the level of the stimulating rays used in the preliminary read-out lower than the level of the stimulating rays in the final read-out the output of the stimulating ray source such as a laser beam source may be decreased in the preliminary read-out, or the stimulating rays emitted by the stimulating ray source may be attenuated by an ND filter, an AOM, or the like, positioned in the optical path. Alternatively, a stimulating ray source for the preliminary read-out may be positioned independently of the stimulating rays source for the final read-out, and the output of the former may be made lower than the output of the latter. Or, the beam diameter of the stimulating rays may be increased, the scanning speed of the stimulating rays may be increased, or the moving speed of the stimulable phosphor sheet may be increased in the preliminary read-out.
When the image information stored in the stimulable phosphor sheet is approximately ascertained prior to the final read-out and the final read-out is conducted by use of the read-out conditions adjusted on the basis of the image information, it becomes possible to eliminate adverse effects of fluctuation in the level of the radiation energy stored in the stimulable phosphor sheet, which is caused by a change in the object or the image recording portion thereof, by a fluctuation in the radiation dose, or the like, and to conduct the final read-out by use of desirable read-out conditions.
When the image processing is carried out by use of the image processing conditions adjusted on the basis of the image information stored on the stimulable phosphor sheet, the image processing can be achieved appropriately in accordance with the object and the image recording portion of the object, and a desirable visible image can always be obtained.
As the method of adjusting the read-out conditions in the final read-out on the basis of the image information obtained by the preliminary read-out, or of adjusting the image processing conditions in the image processing conditions in the image information obtained by the image read-out, the applicant proposed in Japanese Patent Application No. 59(1984)-12658 a novel method comprising the steps of: determining a histogram of the amount of light emitted by a stimulable phosphor sheet in preliminary read-out or in image read-out. calculating the maximum light emission amount Smax and the minimum light emission amount Smin of a desired image information range in the histogram, and adjusting the read-out conditions in final read-out or adjusting the image processing conditions in the image processing so that the maximum light emission amount Smax and the minimum light emission amount Smin correspond respectively to the maximum signal level Qmax and the minimum signal level Qmin of a desired input signal range in the image processing means which are determined by the maximum density Dmax and the minimum density Dmin of a correct density range in the reproduced visible image.
On the other hand, in the case where the radiation image recording and reproducing system is used for medical diagnosis, portions of the human body not related to diagnosis should not be exposed to radiation since the radiation is harmful to the human body. Further, when the human body portions not related to diagnosis are exposed to radiation, the radiation is scattered by such portions to the portion related to the diagnosis, and the contrast and resolution are adversely affected by the scattered radiation. Therefore, in many cases, the irradiation field should be limited when a radiation image is recorded, Normally, when the irradiation field is limited, radiation scattered by the object within the irradiation field passes outside of the irradiation field. The scattered radiation is absorbed and stored in the stimulable phosphor sheet which exhibits high sensitivity, and therefore the histogram of the light emission amount obtained by the preliminary read-out or light emission amount obtained by image read-out includes the light emission amount caused by the scattered radiation. Since the light emission amount caused by the scattered radiation outside of the irradiation field on the stimulable phosphor sheet is often equal to or larger than the light emission amount within the irradiation field, it is not always possible to discriminate between the light emission amounts inside and outside of the irradiation field in the histogram obtained by the preliminary read-out or obtained by image read-out. As a result, when Smax and Smin are calculated from the histogram as described above and the read-out conditions or the image processing conditions are adjusted on the basis of Smax and Smin, the minimum light emission amount within the irradiation field is not detected as Smin. and that caused by the scattered radiation outside of the irradiation field is detected as Smin. In general, the minimum light emission amount outside of the irradiation field is smaller than that within the irradiation field. Accordingly, when the minimum light emission amount outside of the irradiation field is detected as Smin, signals caused by the scattered radiation not related to diagnosis are taken within a low density range in the final read-out or are considered in the image processing. and the density of the image of the portion related to diagnosis becomes too high. As a result, the image contrast decreases and it becomes difficult to make an accurate diagnosis.
Namely, when a radiation image is recorded by limiting the irradiation field, radiation scattered by the object passes outside of the irradiation field on the stimulable phosphor sheet and causes noise in the image information obtained by the preliminary read-out or obtained by the image read-out. Therefore, when the read-out conditions are adjusted based on such preliminary read-out image, it is not always possible to adjust the read-out conditions to appropriate values and to obtain a visible image suitable for viewing, particularly for diagnostic purposes. Similarly, in the case where the image processing conditions are adjusted based on such image information it is not always possible to adjust the image processing conditions to appropriate values and to obtain a visible image suitable for viewing, particularly for diagnostic purposes.